My Tie::Constrained module sets up the sort of thing Roy Johnson mentioned for scalar variables. The pod for it shows several examples.

There are examples on Perlmonks of wedging code into assignment with it. All these do their work even on code written without the need for their special properties in mind.

• Re: Form Validation and Untainting shows how to validate and detaint untrusted data as it is passed through assignment to a trusted variable.
• Lazy DBI Connection Handles shows a global DBI handle which does not connect until needed, and which reconnects if lost.
• To Validate Data In Lvalue Subs was the origin of Tie::Constrained. Assignment is highjacked to permit validation with lvalue closures.